The present invention relates to thermal spray powders and specifically to chromium oxide thermal spray powders.
It is known in the art that it is highly advantageous to apply a fine coating of a ceramic metal oxide to substrates that are, in use, subjected to high temperatures or to highly abrasive or corrosive environments so as to protect the surface of the metal from wear damage or physical deterioration. These coatings can be applied in a variety of ways but among the more frequently used are oxygen fuel guns such as the detonation gun, (the xe2x80x9cD-Gunxe2x80x9d), manufactured and sold by Praxair; and high-velocity oxygen fuel (xe2x80x9cHVOFxe2x80x9d) guns such as those manufactured and sold by Sulzer-Metco, Praxair, TAFA and plasma guns manufactured by SNMI, Sulzer-Metco, Praxair, TAFA and others. In using all such devices, control of the particle size and shape as well as purity are extremely important.
The size parameter is particularly important since a uniform coating is required and because the systems often have trouble handling widely different particle sizes in the same feed. Generally thermal spray powders have particle sizes from 5 to 125 microns but any particular powder used will have a rather narrow particle size variation within this broad range. Thus most applications call for graded sizes such as 5-25, 10-30, 10-38, 15-45, 22-45, 10-63, 45-75, 45-125 and so on. In such graded sizes fines are avoided as far as possible with a fines tolerance usually being specified at no more than 2 to 5% below the bottom limit. Wherever possible particle sizes below 5 microns are removed to increase productivity.
This narrow size distribution is important to optimize melting and delivery of material to the gun at a uniform rate. Fine particles tend to hamper flow and/or cause nozzle build-up during application. They also tend to cause irregular flow and to stick to the surface of larger particles. All this results in uneven coating and build-up rate and when this is detected the process must be shut down and corrective measures taken. If it is not detected a defect, such as the deposition of agglomerates of fine particles, can result in pinholes, large pores or defects which in turn can lead to coating failure or, if detected, require re-working. Thus a narrow particle size range with a minimum of associated fines is critical in providing a high quality thermal spray powder.
The shape of thermal spray powders is also important in their effective and economic use. Rounded shapes are best with blocky shapes also being quite acceptable. However shapes that are weak, that is having a distinct aspect ratio, (the ratio of the longest particle dimension to the next longest dimension perpendicular to the longest), of about 2 or more, can cause severe flow problems and therefore irregular coatings.
In summary the ideal powder for thermal spray applications is round and falls within a narrow size range and has a minimum of fines outside that range. The chemical nature of the powder is determined by the application for which it is intended. Where such application is intended to impart wear resistance, strength, corrosion resistance and suitability for laser engraving for example, the powder of choice is chromium oxide. Such powders have their own specific problem in such applications since they are often contaminated by chromium metal which must be reduced to a minimum, usually below 0.1% by weight, before they can be used.
Chromium oxide is typically produced in a fusion process in an electric arc furnace using a feed of fine pigment-grade chromium oxide with a particle size of about 3 microns. The fusion process causes the particles to melt and fuse into an ingot that is then crushed to the appropriate particle sizes. The arc furnace however, which uses carbon electrodes, operates under somewhat reducing conditions leaving a significant amount of chromium metal as an impurity. This can be reduced to acceptable levels by subsequent calcination under oxidizing conditions. In this process the ingot of fused chromium oxide is milled to produce a fine powder which is then size-classified and the desired particle size range is heated in a furnace at temperatures above about 1100xc2x0 C. in a flow of air. Any resulting agglomeration can readily be broken down to the ultimate particles again by a light milling.
The fusion process is somewhat expensive and results in the loss of the considerable amount of fines, material that is typically removed prior to calcination. These fines are often recycled through the fusion process or otherwise disposed of.
A process for making an improved chromium oxide thermal spray powder has now been devised that is more economical, results in particles with a better shape and involves less unusable by-product. In addition the process has an added degree of flexibility in making surface chemistry adjustments in the particles and therefore the coating applied.
The present invention provides a process for the production of a chromium oxide thermal spray powder which comprises calcining chromium oxide powder with particle size range of 0.1 to 125 microns and comprising at least 5% by volume of chromium oxide particles smaller than 10 microns for a time sufficient to reduce the volume of particles with sizes less than 10 microns to below 5%, and preferably below 2%, of the total weight of chromium oxide.
As indicated above chromium oxide particles are usually made by fusing pure chromium oxide in an electric arc furnace. This is because the fine particles, which are too fine to use directly in thermal spray applications, do not readily sinter together. Rather the surface material tends to volatilize and then condense. This explains the previous commercial practice of fusing, crushing and fines segregation as discussed above..
A preferred process therefore comprises:
a) feeding chromium oxide particles with sizes of from 0.1 to 125 microns into a furnace along with from 0 to 100% based on the weight of the comminuted product, of chromium oxide powder with a particle size less than 10 microns to produce a mixture of particles in which at least 5% by volume of the particles are smaller than 10 microns;
b) calcining the mixture at a temperature above 1000xc2x0 C. for a time sufficient to cause reduction of the content of chromium oxide particles smaller than 10 microns to below 5%; and
c) cooling and classifying the resultant product.
Where the mixture comprises chromium metal, it is preferred that the calcining operation be carried out in an air flow to cause oxidation of the metal to the oxide.
In a further preferred aspect of the invention the initial charge of chromium oxide is obtained by a fusion process producing a block of chromium oxide followed by a comminution of the block until the particle size is below 125 microns. This process has the advantage that the powder obtained by crushing the fused chromium oxide does not need to be classified to remove fines before calcination and, by using the fine powder feedstock used to make the fused chromium oxide powder, the process throughput in greatly increased. At the same time the fines produced by the comminution and the new fine powder added are used to improve the shape of the chromium oxide particles and render them closer to the spherical. This is apparently achieve by volatilization of the fine particles and preferred recondensation of the volatilizate on the larger particles in areas of concavity, thus improving their shapes for thermal spray purposes by making them more spherical. There seems to be a particle size at about 5 microns or larger where particles become sites for condensation or crystal growth. Particles that are less than 5 microns vaporize and then are absorbed or crystallize on the surface of particles that are larger than 5 microns.
Using the fused particles as sites allows the reaction with fine particles to take place at relatively low temperatures above 1100xc2x0 C. and preferred at 1350xc2x0 C. The reaction will also take place when using 100% of particles less than 10 microns providing a protracted time at low temperatures or increased temperature is used. With increased temperature in the range of 1600C., product of 5 to 125 microns from feeds  less than 10 microns can be made in reasonable times without the need for larger fused particles as sites for growth.
It is understood that when reference is made herein to xe2x80x9cparticle sizesxe2x80x9d, these are volume average particle sizes measured using a Leeds and Northrop xe2x80x9cMicrotracxe2x80x9d particle size analyzer which employs a laser light scattering technique to measure the sizes. When running the Microtrac in xe2x80x9cpercent passingxe2x80x9d mode, it is often convenient to describe the particle size distribution in terms of the volume percentage below a given level. Therefore the xe2x80x9cD10xe2x80x9d value is understood to indicate the size where 10 volume % of the particles are smaller than the value at D10 ; D50 indicates the median particle size of the overall sample with equal volumes of particles larger and smaller than the median value; and D90 gives the particle size where 90 volume % of the particles are smaller than that size.